(1) Field of the Invention
The present invention relates to a bidirectional optical transmission system having a light-interruption detecting function.
In a bidirectional optical transmission system, it is required to be able to surely detect a light-interruption state of an optical signal at a receiving party, caused by a cutting or deterioration of an optical transmission line made of an optical fiber, or caused by a malfunction in a transmitting office.
(2) Description of the Related Art
Cross references relating to the present invention are as follows.
(a) Japanese Unexamined Patent Publication No. 59-17637 published on Oct. 6, 1984, which discloses modulating a supervising signal having a single frequency common to all repeaters and different from that of an information-containing light signal, transmitting the signal through a transmission line, receiving it at a terminal unit, separating the information light signal and the supervising light signal, and detecting a fault by monitoring the level of the supervising light signal. PA1 (b) Japanese Unexamined Patent Publication No. 60-144031 published on Jul. 30, 1985, which discloses constant supervision of the characteristics of a transmitter in a bidirectional optical transmission system. At a receiver, a signal from the transmitter in the same office is fed back to the receiver part, and a signal transmitter through an optical fiber is simultaneously input into a light receiving element with the light signal configuration, and a converted electrical signal is separated. Thus, the light output signal from the transmitter part associated with the receiver is constantly supervised. PA1 (c) Japanese Unexamined Patent Publication No. 60-144032 published on July 30, 1985, which discloses demodulation of high-speed pulses into a baseband signal by the use of a pulse modulating method, and passing the baseband signal through a switch, or directly passing the high speed pulse signal through a logical switch. PA1 (d) Japanese Unexamined Patent Publication No. 53-110315 published on Sept. 27, 1979, which discloses a repeater including a supervising pilot signal oscillator. PA1 (e) Japanese Unexamined Patent Publication No. 55-136737 published on Oct. 24, 1980, which discloses a single-frequency supervising pilot signal for supervising all repeaters. PA1 (f) Japanese Unexamined Patent Publication No. 55-136737 published on Oct. 24, 1980, which discloses deteriorated LD information and other information related to the level of received light, which are converted into frequency information and sent to a supervising office for detection of a light repeater in which the LD is deteriorated. PA1 (g) Japanese Unexamined Patent Publication No. 62-245827 published on Oct. 27, 1987, which discloses a fault supervising system in which when a fault occurs, the fault position and the fault contents are simultaneously informed to a terminal without taking the transmission line off line.
In an optical transmission, a bidirectional optical transmission system utilizing a wavelength division multiplexing module or a photo coupler has been considered and developed to attain effective utilization of optical fibers.
Such a wavelength-division multiplexing module or a photo coupler in one office is used to pass a signal transmitted from the one office through a common optical fiber to another office, and also to pass a signal received from the other office through the same common optical fiber to the first office. Thus, only a single optical fiber is used for the bidirectional optical transmission.
The wavelength-division multiplexing module or the photo coupler, however, has a crosstalk characteristic as later described in more detail. Due to the crosstalk in the wavelength-division multiplexing module or the photo coupler, even if a light interruption state occurs in the receiving office or on the optical fiber, it is conventionally difficult to determine whether the signal being received is from the other office or from the same office having leaked through the module or photo coupler into the received part from the transmitter.
None of the above described cross references disclose the problem of crosstalk in the wavelength-division multiplexing module or the photo coupler.